Project 4: Most cancers comprise a heterogenous population of cells with marked differences in their proliferative potential and the ability to reconstitute the tumor after transplantation. There is now compelling evidence that tumors from disparate malignancies harbor a minor population of tumor cells, termed 'cancer stem cells', that possess the unique stem cell property of self-renewal. We hypothesize that clinical non-Hodgkin's Ivmphoma arises from such biologically distinct tumor-initiating cells - a lymphoma stem cell (LySC). The goal of this project is to identify, characterize, and target the tumor-initiating cells found within the context of primary human non-Hodgkin's lymphoma (NHL). In malignancies of differentiated adult tissue such as breast, colon, prostate, and brain cancer, cell populations with the critical stem cell-like properties of selfrenewal and differentiative capacity have been identified. Thus mutation/transformation of a B-lymphocyte could result in the acquisition of functionally defined stem cell properties. Preliminary data derived from NHL specimens present several lines of evidence in support of our hypothesis. In concordance with a central component of stem cell based diseases, we find clear phenotypic and functional heterogeneity in primary and cultured lymphoma specimens. These data strongly support a role for LySC in the pathogenesis of NHL. Further, because cancer stem cells are generally less sensitive to conventional cytotoxic therapy, we propose that the LySC population may represent a central reservoir for cells giving rise to clinical relapse and refractory disease. Therefore, the identification and subsequent targeting of LySC is a critical step toward optimizing therapeutic outcomes in NHL.